Many proposals have heretofore been made to suppress mixed color bleeding on an ink jet recording paper. For example, JP-B-58-72495 (The term "JP-B" as used herein means an "examined Japanese patent publication") and JP-B-59-35977 propose that a large amount of a porous pigment be applied to a base paper to enhance the ink absorption capacity of the base paper. However, when the coating amount of the pigment is increased, it causes a deterioration in the strength of the recording paper. Such a recording paper generates powder falling when bent or rubbed. The powder thus fallen adheres to the running roll, to thereby prevent the roll from running, or to thereby clog the head. Further, this causes a rise in the cost of the recording paper.
On the other hand, JP-A-2-117880 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a recording paper comprising (1) a coating layer applied in a low coating amount having a surface roughness index of not less than 10 ml/m.sup.2 and a surface Beck smoothness of not more than 20 seconds, and (2) an ink-receiving layer having a fibrous substance present in the vicinity of the surface thereof. Further, JP-A-2-117880 proposes a recording paper for ink jet recording method comprising a coating layer applied in a low coating amount having a surface roughness index of not less than 10 ml/m.sup. 2, in which the recording paper has a wetting time of not less than 10 msec as determined by Bristow's method and has a fibrous substance present in the vicinity of the recording surface. However, the coating layers of these recording papers have a rough surface and thus are disadvantageous in that an ink flows into the indented portion of the rough surface to cause mixed color bleeding. Further, since a fibrous substance is present in the vicinity of the ink-receiving layer, the ink penetrability can be easily affected by a base material. Accordingly, when the base material has an unevenness in ink penetrability, there occurs unevenness in an image density of particularly on solid printed areas and halftone printed areas.
Further, JP-A-2-16078 proposes a recording paper for ink jet recording method comprising (1) a base paper and (2) a surface layer applied in a low coating amount comprising a pigment and a fibrous substance contained in the base paper being incorporated into the surface layer in admixture. The recording paper exhibits an initial transition (ink absorption capacity in 10 msec contact of the recording paper with the ink according to Bristow's method) of not less than the maximum recording density (maximum amount of an ink given by the printer). The time during which the recording paper and the ink are brought into contact with each other (10 msec) according to Bristow's method, which defines the initial transition of recording paper, corresponds to the time in which the surface of the recording paper is wet with an ink. This wetting time is drastically affected by the surface roughness of the recording paper. Accordingly, in order to keep the initial transition in the contact time 10 msec greater than the jetted amount of an ink at the maximum recording density, the surface roughness of the recording paper must be raised. As a result, this recording paper is disadvantageous in that an ink flows into the indented portion of the roughness to cause mixed color bleeding, which cannot be avoided. This recording paper is also disadvantageous in that it has a surface layer comprising a pigment and a fibrous substance of base paper incorporated in the surface layer in admixture and thus tends to occur unevenness in an image density for the same reason as described above.
Moreover, JP-A-62-162584 proposes a recording paper mainly composed of wood pulp having a stockigt sizing degree of not more than 40 seconds, an air permeability of not more than 50 seconds and a Beck smoothness of not more than 30 seconds. JP-A-63-1582 proposes a recording paper mainly composed of wood pulp having an air permeability/basis weight ratio of from 0.4 to 2.5. However, since these recording papers are of so-called ordinary type which has no porous pigment present on the base paper, they lack of color developability. Further, the ink penetrability can be easily affected by the base material. Accordingly, when the base material has an unevenness in ink penetrability, the recording paper tends to occur unevenness in an image density particularly on solid printed areas and halftone printed areas.
Further, the following proposals have been made to suppress the unevenness in an image density. JP-A-61-68286 proposes an ink jet recording process comprising printing an aqueous ink on a recording paper, in which the aqueous ink has a water content adjusted to from 10 to 90% by weight to enhance its absorbability, and the recording paper comprises a base paper having a basis weight of from 50 to 90 g/m.sup.2 and a stockigt sizing degree of from 5 to 100 seconds and a coating layer provided on the base paper. JP-A-61-68287 proposes an ink jet recording method with the use of the above-described recording paper and an ink having a viscosity adjusted to not more than 25 cp to enhance its absorbability. JP-A-61-68288 proposes an ink jet recording method with the use of the above-described recording paper and an ink having a surface tension of from 20 to 60 dyn/cm (20.degree. C.), which is close to that of the recording paper, to predetermine the spread of dot. These methods are featured by the combination of specific recording paper and specific ink. However, the recording paper which is characterized only in the basis weight and st6ckigt sizing degree of the base paper cannot drastically suppress the unevenness in an image density.
JP-A-62-216785 proposes a recording paper composed of an uncoated base paper which exhibits a specified peak wavelength in the power spectrum of the shape of the recording surface in order to enhance the ink absorption capacity and improve the dot shape. However, since this recording paper has no coating layer, it exhibits a reduced ink absorption capacity and an insufficient color developability and resolution on the printed area. Further, since the ink is directly absorbed by the base paper, the ink penetrates into the base paper along fibers, to thereby cause mixed color bleeding and a poor dot shape. As a result, unevenness in an image density cannot be reduced sufficiently.